Small-sized portable devices are widespread worldwide these days, and at the same time demand for small-sized and large-capacity nonvolatile memory is rapidly expanding with significant progress in high-speed information transmission network. Among such memories, NAND flash memory and small-sized HDD (hard disk drive) have made rapid progress in recording density and form a big market.
Under such a situation, some ideas of new memory that aim to greatly exceed the limit of recording density are proposed. For example, PRAM (phase-change memory) uses a material capable of having two states, an amorphous state (OFF) and a crystal state (ON), as a recording material, and uses a method that relates the two states to two-value data “0” and “1” to record data.
In regard to writing/erasing, for example, a high-power pulse is applied to the recording material to produce the amorphous state, and a low-power pulse is applied to the recording material to produce the crystal state.
The readout is performed by passing a readout current small enough not to cause writing/erasing through the recording material and measuring the electric resistance of the recording material. The resistance value of the recording material in the amorphous state is larger than the resistance value of the recording material in the crystal state, and the ratio thereof is about 103, for example.
Furthermore, also a memory is reported that uses changing the resistance by a method different from that of the PRAM. For example, there are: a memory layer including a high resistance film and an ion source layer (JP-A 2007-80311 (Kokai)); and a variable resistance device including a conductor film and an insulator film (JP-A 2007-299436 (Kokai)). In these memories, an ion is used, and a metal element is ionized or an ionized metal element moves; thereby, the resistance value of the memory device changes. In the former, the ion source layer contains: one or more elements (metal element) selected from Ag, Cu, and Zn; and one or more elements (chalcogenide element) selected from S, Se, and Te. In the latter, as the material of the conductor film, for example, a metal film, alloy film (e.g. CuTe alloy film), metal compound film, and the like containing one or more metal elements selected from Cu, Ag, and Zn are given.